A known vehicle motion control apparatus is disclosed in Japanese Patent Application No. 2000-203401A (hereinafter, referred to as reference 1). According to the motion control apparatus disclosed in reference 1, shutoff valves are arranged to be in a closed position and intake valves are arranged to be in an opened position. Further, hydraulic pressure pumps are operated and output brake hydraulic pressure of the hydraulic pressure pumps are supplied to a wheel cylinder, and so on, thus performing an automatic pressurization. Then, the smaller a frictional coefficient of a road surface is, the rotational speed of a motor lowers. In the mean time, the rotational speed of the motor lowers in accordance with an actual control amount approaching a target control amount. Consequently, an operational amount of each of the hydraulic pressure pumps is reduced and an operational noise is reduced.
According to the motion control apparatus disclosed in the reference 1, the smaller the frictional coefficient of the road surface is, the lower the rotational speed of the motor is arranged, thus leading to reduce the operational noise of the motor. However, when the rotational speed of the pump is reduced, a discharging amount of each pump is reduced. Accordingly, a pressure increasing rate relative to the wheel cylinder lowers under a target pressure increasing rate, which corresponds to a pressure increasing rate required for controlling a vehicle behavior. Therefore, there is a possibility that a pressure-increasing responsiveness may not be obtained properly.
A need thus exists for a vehicle motion control apparatus which is not susceptible to the drawback mentioned above.